In internal combustion engines, camshafts are used for actuating the gas-exchange valves. The camshaft is mounted in the internal combustion engine in such a way that cams attached to it bear against cam followers, for example bucket tappets, drag levers or rocker arms. When the camshaft is set in rotation, the cams roll on the cam followers which, in turn, actuate the gas-exchange valves. Thus, by virtue of the position and shape of the cams, both the opening duration and the amplitude, but also the opening and closing time point of the gas-exchange valves, are defined.
Modern engine concepts tend towards designing the valve drive variably. On the one hand, the valve stroke and the valve opening duration are to be capable of a variable configuration up to the complete cut-off of individual cylinders. For this purpose, concepts, such as switchable cam followers, variable valve drives or electrohydraulic or electric valve actuations are provided. Furthermore, it has proved advantageous to be capable of influencing the opening and closing times of the gas-exchange valves while the internal combustion engine is in operation. It is likewise desirable to be able to influence the opening or closing time points of the inlet or outlet valves separately, so that, for example, a defined valve overlap can be set in a purposeful way. By the opening or closing time points of the gas-exchange valves being set as a function of the current characteristic diagram range of the engine, for example of the current rotational speed or the current load, the specific fuel consumption can be lowered, exhaust-gas behaviour can be influenced positively and the engine efficiency, maximum torque and maximum power can be increased.
The described variability in the time control of the gas-exchange valves is brought about by means of a relative change in the phase position of the camshaft with respect to the crankshaft. In this case, the camshaft is drive-connected to the crankshaft mostly via a chain, belt or gearwheel mechanism or via equivalent drive concepts. Between the chain, belt or gearwheel mechanism driven by the crankshaft and the camshaft, a camshaft adjuster is mounted, which transmits the torque from the crankshaft to the camshaft. In this case, this device for varying the control times of the internal combustion engine is designed in such a way that, while the internal combustion engine is in operation, the phase position between the crankshaft and camshaft can be held reliably, and, if desired, the camshaft can be rotated over a particular angular range with respect to the crankshaft.
In internal combustion engines with a camshaft in each case for the inlet and the outlet valves, these may be equipped in each case with a camshaft adjuster. As a result, the opening and closing times of the inlet and outlet gas-exchange valves can be displaced relative to one another in time and the valve time overlaps can be set in a purposeful way.
The seat of modern camshaft adjusters is generally located at the drive-side end of the camshaft. It consists of a driving wheel fixed with respect to the crankshaft, of a driven part fixed with respect to the camshaft and of an adjusting mechanism transmitting the torque from the driving wheel to the driven part. The driving wheel may be designed as a chain wheel, belt wheel or gearwheel and is connected fixedly in terms of rotation to the crankshaft by means of a chain, a belt or a gearwheel mechanism. The adjusting mechanism may be operated electromagnetically, hydraulically or pneumatically. It is likewise conceivable to mount the camshaft adjuster on an intermediate shaft or to mount it on a non-rotating component. In this case, the torque is transmitted to the camshafts via further drives.
Electrically operated camshaft adjusters consist of a driving wheel which is drive-connected to the crankshaft of the internal combustion engine, of a driven part which is drive-connected to a camshaft of the internal combustion engine, and of an adjusting gear. The adjusting gear is a triple-shaft gear with three components rotatable with respect to one another. In this case, the first component of the gear is connected fixedly in terms of rotation to the driving wheel and the second component is connected fixedly in terms of rotation to the driven part. The third component is operatively connected to the first and the second component, for example by means of pairs of toothings, articulated levers or friction-wheel pairings. The rotational speed of the third component is regulated, for example, by means of an electric motor or a braking device. By means of different numbers of teeth of the toothings of the three components, lever kinematics or different diameters of the friction wheels, a transmission ratio between the first and the second component which is unequal to 1 is implemented. The phase position can thereby be selectively held or varied by the choice of suitable rotational speeds of the third component.
The torque is transmitted from the crankshaft to the first component and from there to the second component and consequently to the camshaft. This takes place either directly or with the third component being interposed.
Via suitable regulation of the rotational speed of the third component, the first component can be rotated with respect to the second component and consequently the phase position between the camshaft and crankshaft can be varied. Examples of triple-shaft gears of this type are internal eccentric gears, double internal eccentric gears, harmonic drives, swashplate mechanisms, epicyclic gears, tungsten gears or the like.
To control the camshaft adjuster, sensors detect the characteristic data of the internal combustion engine, such as, for example, the load state, the rotational speed and the angular positions of the camshaft and crankshaft. These data are fed to an electronic control unit which, after comparing the data with a characteristic diagram of the internal combustion engine, controls the adjusting motor of the camshaft adjuster.
DE 102 48 355 discloses a device for varying the control times of an internal combustion engine, in which torque transmission from the crankshaft to the camshaft and the adjusting operation are carried out by means of a double epicyclic gear. The torque of the crankshaft is transmitted to a driving element of the device via a chain mechanism. The driving element is designed as a ring wheel, the internal toothing of the ring wheel meshing with external toothings of a plurality of planet wheels arranged on a planet carrier and designed as spur wheels. The external toothings of the spur wheels engage simultaneously into an internal toothing of a driven element which is designed as a ring wheel and which, in turn, is connected fixedly in terms of rotation to a camshaft. Furthermore, the toothings of the planet wheels mesh with an external toothing of a sun wheel which serves as an adjusting shaft and is driven by an electric motor. The phase position between the driving element and driven element is held or adjusted as a function of the rotational speed of the electric motor. So that the phase position of the two components can be varied, the driving element is mounted on the driven element rotatably with respect to the latter by means of a plain or rolling bearing.
The driving element is designed in the axial direction with a shoulder, by means of which it is supported in one axial direction on the driven element. The driving element is likewise supported on the driven element in the other direction by means of a spring ring. In this embodiment, the mountings are designed as plain bearings.
The adjustment of devices of this type is carried out via electric drives which regulate the rotational speed of an adjusting shaft. In order to design the electric drive cost-effectively and so as to be optimized in terms of construction space, a high efficiency of the device is desirable. A precondition for a high efficiency is minimal friction between the components of the device. In this regard, the plain-bearing mounting of the driving element with respect to the driven element has proved to be a disadvantage, especially in the case of high adjustment speeds and high tilting moments acting on the driving element.